U.S. Provisional Application No. 62/031,505, filed Jul. 31, 2014, is incorporated herein by reference in its entirety.
In electric arc furnace plants, metal scrap is melted in a batch furnace and tapped into a transfer ladle. Within this ladle, further alloying agents can be added, vacuum degassing can occur, along with further heating to maintain metal temperature. The molten steel can be held in the ladle from approximately 60 to 300 minutes depending on various factors. When the ladle is to be emptied, the slide gate is shifted allowing holes in both plates to align and metal flow to begin. The current standard practice in the steel industry is to fill the upper cavity of the nozzle and well block assembly with a graded blend of refractory sands to avoid freezing of the molten steel, causing a non-free open of the ladle. The molten steel contacts the sand bed and forms a composite sand-steel ‘interfacial zone.’When the slide gate is opened, the loose sand flows out and the ferrostatic head pressure of the steel breaks through the interfacial zone and into the receiving tundish. As the time the metal is held in the ladle increases, permeation of the steel into the sand bed increases and this ‘interfacial’ zone thickens. This often results in a non-free open where operators use a stainless steel pipe with oxygen to burn through the interface and allow the steel to flow.
The current industry standards for ladle nozzle sand revolve around four main components; silica sand, zircon sand, chromite sand, and a fine carbon grain. These are blended in various ratios depending on the practice at the individual mill. The most active species is usually considered to be the zircon sand although its use is being eliminated in favor of chromite sand in some cases. The general rule is that as the hold times of the molten steel in the ladle increases, the zircon content is increased to improve the chances of free opening. There are drawbacks to zircon sand's use in that it will adhere to the well and nozzle refractories over time and can cause wear as well as a need for mechanical removal when the ladle is prepared for another charge. Also, historically zircon supply and pricing has fluctuated dramatically based on global demand. The chromite sand is used to counter some of the effects of zircon sand and as a lower cost alternative completely replacing zircon in some instances. Chromite sand is also a relatively expensive commodity item and is subject to commercial fluctuations. The silica sand is a low cost filler material and the fine carbon is used to limit permeation of the steel into the sand bed and to reduce some of the corrosive effects of the steel on the well and nozzle assembly.